Spline IK bake and export problem
This is a scaling artifact that happens when Y-stretch is enabled. The bone scales get applied additively down the chain, because the bones are parented to each other, resulting in an odd shape.

What you need to do is: Copy the SplineIK bone chain, delete the SplineIK constraint in this new chain and unparent all the bones from each other. Than add copy transform constraints to every bone in the chain, targeting the matching bone in the SplineIKchain. Make this new chain deform the mesh, disable deform for all the bones in the original splineIK bone chain and enable only deform bones in export.